Intermolecular hetero Diels-Alder reactions of enamino ketones with highly substituted vinyl ethers effect of high pressure on the kinetics and diastereoselectivity

The hetero Diels-Alder reactions of enamino ketones 1 and 2 with the vinyl ethers 3-5a are studied in dichloromethane solution under high pressures up to 5 kbar. The kinetics is measured by on-line FT-IR spectroscopy up to 3 kbar. The cycloaddition reactions of enamino ketones 1 and 2 with vinyl ethers 3 and 4 yield the diastereomeric dihydropyrans 7a-d and 8a-d, respectively, whereas in the cycloaddition of enamino ketone 1 to vinyl ether 5a the constitutional isomers 9 and 10 are formed. The unexpected reaction of 1 with 5a to give the spiro compound 9 is explained by isomerisation of 5a to 5b under the reaction conditions. The kinetics of the reactions is discussed in terms of both overall and individual rate coefficients, activation energies, and activation volumes.